A novel IF frequency stabilization scheme for coherent optical FDM systems

An IF frequency stabilization scheme for coherent multichannel systems was demonstrated. The transmitter laser diodes are frequency-modulated with sinusoidal signals for channel spacing stabilization. A frequency-selective IF servo-system consisting of a phase-locked loop and a synchronous filter was used to make the local oscillator track the additional sinusoidal modulation on the transmitter frequency so that the modulation amplitude in the IF domain was greatly reduced. In the demonstration experiment with frequency-shift-keying (FSK) transmission, it was reduced to 5 MHz/sub p-p/ and no system performance degradation due to this residual modulation was observed. The circuit also gives a channel identification signal.<>